My Mac Hard Drive space just keeps draining?

Hey all, Today i logged on my mac, I just thought i'd have a look around my files, And had 33 Gigs of space left, So i turned of my macbook and came back to to it a few hours later, Turned it on and i just watched my harddrive space go down from 33G down to Zero, Help?! I've tryed all theese app's to show where my disk space ect.. has gone, Nothing comes up, I've ran IAntiVirus, No Virus's. I really do not know what to do :/ Please Help Many Thanks, Jon.

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